This invention relates generally to polycarbonate plastics, and more particularly to the preparation of high molecular weight substituted aromatic polycarbonates from sterically hindered monomers.
The preparation of substituted aromatic polycarbonates by interfacially polycondensing substituted aromatic dihydroxy compounds and phosgene is known. In this reactin, phosgene is passed into a well-stirred two-phase mixture of an aqueous alkaline solution containing the substituted aromatic dihydroxy compound and a polycarbonate solvent such as, for example, methylene choloride. It would be desirable to prepare high molecular weight substituted aromatic polycarbonates using solid or liquid raw materials, thereby eliminating the use of phosgene which is an extremely poisonous gas.
While interfacial polymerization processes, utilizing phosgene and dihydroxy compounds, are generally effective in producing substituted aromatic polycarbonates, they do suffer from a general difficulty in producing high molecular weight, i.e., having a relative viscosity (n.sub.rel) greater than about 1.2, substituted aromatic polycarbonates, because of the steric hinderance caused by the substituent groups. It is, therefore, economically advantageous to promote such reactions to produce high molecular weight substituted aromatic polycarbonates, without having to employ more severe reaction conditions, by the use of a catalyst in the interfacial polymerization process.
The prior art discloses in U.S. Pat. No. 3,271,364, that certain quaternary ammonium compounds, such as, for example, triethylbenzylammonium chloride, catalyze the polycondensation of bischloroformates. Triethylbenzylammonium chloride will not, however, polymerize sterically hindered, substituted aromatic bischloroformates to give high molecular weight polycarbonates.
U.S. Pat. No. 3,912,687 teaches the preparation of substituted aromatic polycarbonates, utilizing phosgene and o,o,o',o'-tetro-substituted bisphenols, by an interfacial process using triethylamine as a catalyst. However, the process results in relatively low molecular weight products. Additionally, this process requires a long reaction time and large amounts of the catalyst.
The use of a substituted pyridine is taught in U.S. Pat. Nos. 4,286,085; 3,428,600 and 3,530,094, as a catalyst in the reaction of a dihydroxy phenol with a carbonate precursor such as phosgene or a bischloroformate. However, that reference does not suggest that a substituted pyridine would be an effective catalyst for the self condensation of a bischloroformate of a sterically hindered, substituted diphenol.
Accordingly, advantages of the present invention are that it provides an industrially advantageous process for the production of high molecular weight polycarbonates, using easily handleable raw materials, and avoiding highly toxic precursors such as, for example, phosgene.
Other advantages of the present invention will become apparent from the detailed description to follow.